In a vehicle engine, a change in torque may occur for example due to a pulsation in the intake air supply. In the high load region of the engine, the resistance of the throttle on intake air is small. The torque change due to this intake pulsation therefore becomes larger, and "surging", wherein the vehicle vibrates at low frequencies, tends to occur.
This surging is assisted by feedback control of the engine air-feel ratio. This is because feedback control of the air-fuel ratio causes the air-fuel ratio to oscillate with a predetermined period around a target value.
However if feedback control of the air-fuel ratio is not performed in the high load region of an engine, deviations of the air-fuel ratio, which occur due to production errors and time-dependent deterioration of the fuel injection mechanism such as a fuel injection valve and an air flow meter, are not corrected.
A three-way catalyst which purifies exhaust from the engine performs optimally in the vicinity of the stoichiometric air-fuel ratio. If the air-fuel ratio shifts to rich from the stoichiometric air-fuel ratio, the ability of the three-way catalyst to oxidize hydrocarbons (HC) and carbon monoxide (CO) in the exhaust falls. If the air-fuel ratio shifts to lean from the stoichiometric air-fuel ratio, the ability of the three-way catalyst to reduce nitrogen oxides (NOx) in the exhaust falls. In both cases, harmful components of the exhaust increase.
A possible method of achieving the dual objectives of preventing surging in the high load region of the engine and preventing deviation of the air-fuel ratio, is to introduce learning control instead of feedback control of the air-fuel ratio. Air-fuel ratio feedback control is performed in an engine running region except the high load region, and using an air-fuel ratio correction coefficient learned in this case, air-fuel ratio in the high load region is controlled by an open loop.
The air-fuel ratio correction coefficient in regions other than the high load region is not always effective for correcting a deviation of the air-fuel ratio in the high load region however. In particular, when the flowrate characteristics of the fuel injection valve and air flow meter vary largely in the high load region and other regions, the deviation cannot be covered by a single air-fuel ratio correction coefficient.